First. I know normal physics don't apply to a fantasy game but lets just play with the thought.
Lets take a look at a Quickling Monk level 18/Sorc level 1. (The Quickling has the run feat & Expeditious Retreat)
Expetidious Retreat => Run speed = 7200' per round
1 foot = 30,48 cm
1 kilometers/hour = 0.62137 miles/hour
1 Mach [dry air, 273 Kelvin] = 33162.2 centimeters/second
Lets make an assumption
1 round = 6 sec
So that would mean:
7200' per 6 sec = 36576 cm/sec = 1316,736 km/h = 818,2 miles/hour
Quickling Monk/Sorc Speed = 1,1 Mach
And this is without adding the extra partial action Quicklings normally have. (With the extra partial action the speed would almost be 1,7 Mach )
The normal running speed for a human (no feat) would in this system (D&D) be:
120'/ 6sek = 609,6 cm/sek = 22 km/h = 13,6 miles/hour (Edited from 13,7 miles/hour)